The present invention generally relates to intravascular ultrasound (IVUS) imaging. The present invention more specifically relates to IVUS systems for co-registered imaging.
Intravascular ultrasound imaging is generally performed to guide and assess percutaneous coronary interventions, typically the placement of a bare-metal or drug-eluting stent. Other applications of IVUS imaging comprise further assessment of coronary artery disease.
Coronary stents generally have struts made of a metal, such as stainless steel or a cobalt chromium alloy. The metal stent struts provide a substantially larger reflected ultrasound signal than blood and soft tissue, such as neotissue grown over stent struts. The ability to detect and measure neotissue growth is particularly relevant for evaluating the stent healing process. Current commercially available IVUS systems have limited ability to detect early neotissue growth, because of a limited detectable range of reflected ultrasound signals.
Atherosclerotic lesions that are prone to rupture, so called vulnerable plaques, are of increasing interest to interventional cardiologists. One type of vulnerable plaque thought to be responsible for a large percentage of plaque ruptures is a thin-cap fibroatheroma wherein a thin (<65 μm) fibrous cap overlies a mechanically unstable lipid-rich or necrotic core. Current commercially available IVUS systems operate up to only 40 MHz and have axial resolutions that are limited to approximately 100 μm. Consequently, current commercially available IVUS systems cannot reliably detect vulnerable plaques.
It is generally necessary to increase the imaging frequency in order to improve spatial resolution. However, increased imaging frequency also leads to reduced contrast between blood and non-blood tissue that in turn makes difficult segmentation of the blood-filled lumen from the intimal plaque. Some automatic segmentation algorithms exploit the frequency-dependent ultrasound properties of blood and non-blood tissues as described for example in U.S. Pat. No. 5,876,343 by Teo. Real-time, automatic segmentation tools are often prone to errors which reduce their utility in clinical practice.
Multi-frequency imaging has been developed for transthoracic echocardiographic applications. U.S. Pat. No. 6,139,501 by Roundhill et al. describes a system that simultaneously displays two B-mode images of different imaging frequencies and bandwidths. However, this technique uses both fundamental and harmonic imaging techniques and relies upon non-linear propagation properties of tissue. Although harmonic imaging can potentially provide better spatial resolution, harmonic imaging performance in the near-field is limited. Further, harmonic IVUS imaging has not been found to be practically useful.
Multi-frequency IVUS imaging can also be achieved by use of multiple transducer imaging catheters. However, multiple transducers add complexity and cost to the disposable imaging catheter and the imaging system. The potential need to co-register the images from the separate transducers further complicates their practical use.
There exists a need for a technology that provides sufficient contrast resolution to guide percutaneous coronary interventions and sufficient contrast and spatial resolution to detect stent healing and vulnerable plaques. Further, it is desirable that such a technology does not require any co-registration step between multiple images. Still further, it is desirable that such a technology does not substantially increase system and catheter complexity and cost over existing commercial systems and catheters.